resin-ee Collections: The @OneToMany Relation

The @OneToMany relation adds collection extensions to the query language and provides a Java Collection containing the children. @OneToMany represents a collection of children belonging to a parent, like students in Gryffindor house at Hogwarts school.

The Many-To-One tutorial illustrated that a many-to-one relation links one source entity to another target entity. A one-to-many relation links the target entity back to the source entity.

In this example, each House has many Students, each Student has one House. House has a one-to-many relationship with Student, Student has a many-to-one relationship with House

tutorial one2many
web.xml configuration persistence.xml configuration The student bean The house bean The test servlet

The database schema is unchanged from the Many-To-One tutorial, and might look like:

CREATE TABLE house ( id BIGINT PRIMARY KEY auto_increment, name VARCHAR(250), ) CREATE TABLE student ( id BIGINT PRIMARY KEY auto_increment, name VARCHAR(250), house BIGINT REFERENCES house(id) )

The bean has a students field that returns the students that belong to the house. House annotates getStudents with @OneToMany to describe the relationship. The @JoinColumn is used to specify the foreign key, the field in Student that is the @ManyToOne link.

@OneToMany(mappedBy="house") private Set<Student> _students;

A @OneToMany always requires a corresponding @ManyToOne on the target entity. The Student has a house field annotated a @ManyToOne, unchanged from the Many-To-One tutorial.

@ManyToOne @JoinColumn(name="house") private House _house;

The one-to-many relation provides a House the ability to get all of its Students. Resin will perform any necessary database lookup.

The example queries all houses and prints their names and all of their students.

private void doService(PrintWriter out) throws { public void service(HttpServletRequest req, HttpServletResponse res) throws, ServletException { PrintWriter out = res.getWriter(); res.setContentType("text/html"); String sql = "SELECT h FROM House h"; Query allHouse = _entityManager.createQuery("SELECT o FROM House o"); List houses = allHouse.getResultList(); for (int i = 0; i < houses.size(); i++) { House house = (House) houses.get(i); out.println("<h3>" + house.getName() + "</h3>"); for ( Student student : house.getStudents() ) { out.println( student.getName() + "<br>" ); } } } }
SELECT h.students FROM House h WHERE'Gryffindor'
SELECT s FROM House h, IN(h.students) s WHERE'Gryffindor'
SELECT s FROM Student s, House h WHERE s MEMBER OF h.students
SELECT h FROM House h WHERE h.students IS EMPTY